WordPress does not have the ability to add Tags and Categories to Pages by default. This is sometimes useful in order to filter pages easily using the Oxygen Repeater Element.
Paste the Below Code in the PHP section of your favourite code editor and set Visibility to everywhere, no need for an extra plugin.
Add Categories to Pages
function add_categories_to_pages() {
register_taxonomy_for_object_type( 'category', 'page' );
}
add_action( 'init', 'add_categories_to_pages' );Add Tags to Pages
function add_tags_to_pages() {
register_taxonomy_for_object_type( 'post_tag', 'page' );
}
add_action( 'init', 'add_tags_to_pages');
If you want an easier option you can download this plugin.
https://wordpress.org/plugins/add-category-to-pages/